Fix negative indices on empty array don't affect next chosen index

UPGRADING

@@ -39,6 +39,8 @@ PHP 8.3 UPGRADE NOTES
     inherited from the parent class. This will create a separate static property
     storage for the current class. This is analogous to adding the static
     property to the class directly without traits.
+  . Assigning a value to a negative index n in an empty array will now make sure that
+    the next index is n+1 and no longer default to 0.
 
 - FFI:
   . C functions that have a return type of void now return null instead of

Zend/zend_hash.c

@@ -255,7 +255,7 @@ ZEND_API const HashTable zend_empty_array = {
 	.nNumOfElements = 0,
 	.nTableSize = HT_MIN_SIZE,
 	.nInternalPointer = 0,
-	.nNextFreeElement = 0,
+	.nNextFreeElement = ZEND_LONG_MIN,
 	.pDestructor = ZVAL_PTR_DTOR
 };
 

ext/standard/tests/array/negative_index_empty_array.phpt

@@ -0,0 +1,18 @@
+--TEST--
+Test empty arrays with first added index being negative
+--FILE--
+<?php
+
+$a = array();
+$a[-5] = "-5";
+$a[] = "after -5";
+
+var_dump($a);
+?>
+--EXPECT--
+array(2) {
+  [-5]=>
+  string(2) "-5"
+  [-4]=>
+  string(8) "after -5"
+}